The disclosure relates to diamond Schottky diodes and methods of manufacturing diamond Schottky diodes.
Modern semiconductors are typically based on silicon, with various elements doped to change their electrical properties. For example, doping silicon with phosphorous creates a surplus of electrons resulting in n-type semiconductor material due to the fifth valence electron not present in silicon, which has only four valence electrons. Similarly, doping silicon with boron creates p-type silicon having a surplus of “holes”, or an absence of electrons, because boron has only three valence electrons which is one fewer than silicon. Silicon can be doped by diffusion or ion implantation, as is known in the art. When n-type and p-type silicon are in contact with one another, electricity flows in one direction across the junction more easily than in the other direction. More complex configurations of n-type and p-type material can be assembled to form various types of transistors, integrated circuits, and other such devices.
The Schottky potential barrier at a metal/semiconductor contact is ideal for fast switching, and low voltage drop in the forward conduction mode reduces the power dissipation of the device. Therefore, the Schottky diode is a preferred over the PN junction diodes in many power applications. When high voltage and high current are applied to a Schottky diode, the energy dissipated in the device leads to an increase in temperature. At room temperature, the reverse current is decided by the field emission mechanism in which the current tunnels through the Schottky barrier. At high temperatures, the thermal field emission mode becomes dominant and not only the tunneling current, but also carriers having heat energy pass over the Schottky barrier as a reverse leakage current. It is therefore necessary to control the accumulation of heat in Schottky diodes employed in high power applications. The effectiveness of cooling systems is limited by the thermal conductivity of the materials from which the diode is constructed.
Silicon (Si) has a thermal conductivity of 1.5 W/cmK and Silicon Carbide (SiC) has a thermal conductivity of 4.9 W/cmK. Diamond has a thermal conductivity of 22.0 W/cmK, which is the highest value among all materials, making diamond very attractive as a material for power handling semiconductor components.
In addition to high thermal conductivity, diamond is a semiconductor with electrical properties suitable for manufacture of devices such as Schottky diodes, and superior to Silicon or Silicon Carbide. For example, diamond has a band gap of about 5.47 eV, compared to 1.1 eV for Si and 3.2 eV for (4H) SiC. On account of this band gap and the associated high critical field strength of about 10 MVcm−1 and low intrinsic charge carrier concentration which, even at temperatures above 300° C., is still markedly below typically used doping concentrations, unipolar power components having reverse strengths of markedly above 100 kV and simultaneously low static and dynamic losses can be produced on the basis of diamond. Due to this, it would be possible to replace the series-connected bipolar power rectifiers on the basis of silicon, which are used in this voltage range, e.g. in the transmission of high voltage direct current, with unipolar power rectifiers on the basis of diamond. This would considerably reduce the costs which are required to cool the bipolar power rectifiers on the basis of silicon and the weight and the volume for the power-electronic components, e.g. in the transmission of high voltage direct current. Another advantage which results from the low intrinsic charge carrier concentration of diamond is the fundamental applicability at temperatures of markedly above 500° C.
To create a functional diamond-based unipolar power diode, it is necessary to limit the electric field at the boundary between the diamond substrate and the Schottky metal. Without such a limitation, the effective Schottky barrier would be markedly reduced when a reverse voltage is applied and the reverse current would increase significantly. Ultimately, this would heat the power diode, and cause the thermal destruction of the diode in extreme cases. Such a limitation is absolutely necessary for an actual use of unipolar power diodes on the basis of diamond due to the critical field strength which is high compared to silicon and silicon carbide. The challenge is now to realize an appropriate component structure in order to limit the electric field.
The current state of manufacturing technology has limited the production of diamond-based power components. Specifically, it has only recently become possible to manufacture diamond substrates with low defect density and dimensions needed for component manufacture.
Nevertheless, the fundamental functional principle of unipolar power diodes on the basis of diamond has already been shown by way of experiment. The diamond based semiconductor devices typically have a first p-doped diamond region, a p-doped drift region having a thickness of about 10 μm and a boron doping of about 1×1016 cm−3 as well as an Au/Mo and/or Au/Ru coating. The Au/Mo and/or Au/Ru coatings here form a potential barrier with respect to the p-doped drift region, which results in a threshold voltage of about 1.2 V. With an operating temperature of 250° C. it is possible to achieve forward voltages within the range of about 2.5 V with the present structure for a diamond Schottky diode when the forward current density is 100 Acm−2. Considering the nominal reverse voltage of about 2 kV, which results from the thickness and doping of the drift region, the shown forward voltage highlights the fundamental advantages which diamond has over comparable semiconductor materials.
Experimental results for the electric characterization of the barrier properties show a marked increase in the reverse current when reverse voltages are already markedly below the maximum reverse voltage of 2 kV. This increase in the reverse current which, with a reverse voltage of only 300 V, already leads to a loss of the barrier capacity of the unipolar power diode on the basis of diamond, can be ascribed, on the one hand, to defects in the crystal layer and, on the other hand, to the need to limit the electric field at the boundary between the diamond substrate and the Schottky metal. The described problem has not yet been specified with unipolar power diodes on the basis of diamond.
As to unipolar power diodes on the basis of silicon or silicon carbide, the described problem is typically solved by introducing so-called JBS areas. N-doped regions which are introduced into a p-doped drift region by means of a method for selective doping (diffusion, ion implantation). When a negative reverse voltage is applied to the cathode, the space-charge region (SCR) spreads between the p-doped JBS strip into the n-doped drift region and narrows the transitional region between the JBS strip. Due to this narrowing, the Schottky metal is shielded from the electric field, and the resulting reduction in the effective Schottky barrier is markedly reduced. However, it is not easy to apply the described method to unipolar power diodes on the basis of diamond. This is substantially due to the limited doping capacity of diamond. In contrast to Silicon or Silicon carbide, selective doping methods, such as diffusion and implantation, cannot be used in the diamond technology and the diamond can only be doped during the crystal growth. Therefore, the realization of a unipolar power diode on the basis of diamond calls for a novel concept in order to realize the JBS areas.
There is a need in the art for methods of manufacturing JBS areas on diamond semiconductor material.